1. Field of the Invention
The present invention relates to an optical element suitably applied to a resin lens and the like which is used in an optical device, and to a method for manufacturing the optical element.
2. Description of the Related Art
Recently, there has been an increasing demand for lenses made of resin which can be manufactured at low cost for use in optical devices, such as an optical pickup device and the like. Such lenses are usually manufactured by resin molding using molds.
FIG. 8 is a plan view showing a conventional lens 50, manufactured by molding, before gates 64 are cut; FIG. 9 is a sectional view taken along the line 9xe2x80x949 in FIG. 8; and FIG. 10 is an illustration for explaining the state of resin 70 filled in a cavity 65.
The conventional lens 50 is resin-molded using molds 60 and 61, as shown in FIG. 9. The molds 60 and 61 include a sprue 62, which is an injection port for the resin, runners 63 equivalent to passages for the resin, the cavities 65 in which the lenses 50 are molded, and the gates 64 equivalent to inlet ports from the runners 63 to the cavities 65.
Each gate 64 is provided at a position of each cavity 65 corresponding to a part of a side wall surface (outer peripheral surface) of each lens 50, and the resin 70 is filled in each cavity 65 using each gate 64 as an inlet port. That is, as shown in FIG. 10, when the resin 70 is filled in the cavity 65, the resin flows approximately in one direction from the gate 64 toward the sidewall surface of the cavity 65 opposing the gate 64. When the cavity 65 is completely filled with the resin 70, the resin 70 is cooled and the lens 50 is completed.
The lens 50 manufactured as described above, however, produces a strain approximately in one direction (x-direction) along the flow of the resin 70 and as a result, for example, so-called astigmatism is caused in which the focal position of light in the x-direction does not coincide with that in the direction perpendicularly intersecting the x-direction, and the lens has directional properties. For this reason, when the lens 50 is mounted in an optical device, the orientation of the lens 50 around an optical axis must be fixed, for example, after minimizing the astigmatism of the lens 50 when used in an optical system of the optical device. This causes a reduced assembly efficiency.
Accordingly, it is an object of the present invention to provide an optical element which does not have directional properties around an optical axis, and a method for manufacturing the optical element.
According to an aspect of the present invention, there is provided an optical element having an optical axis, wherein the optical element is formed by resin-molding, the position of the optical axis at a transmission surface or a reflection surface of light is set at the position of a gate serving as an inlet for resin, and a light-shielding zone for blocking the transmission or reflection of light is provided at the position of the gate.
Since the optical element is formed by the resin which is radially filled from the gate formed at the position of the optical axis, that is, filled with rotational symmetry with respect to the optical axis, astigmatism, which occurs in a conventional lens formed by filling resin in one direction, does not occur, and the optical element can be prevented from having directional properties. In addition, when the optical element is mounted in an optical device, the optical element may be fixed with little concern for the position of the optical element around the optical axis, so that assembly efficiency can be increased. Furthermore, when the optical element is, for example, a lens, the diameter of a focussed spot can be reduced using the light-shielding zone without shortening the wavelength of the light.
In the above optical element, the light-shielding zone may preferably be a light-shielding plane formed by a surface rougher than the transmission surface or the reflection surface. With this arrangement, the optical element can be easily constructed at low cost.
In addition, in the above optical element, the light-shielding plane may preferably be formed to protrude from the transmission surface or the reflection surface. With this arrangement, for example, when the resin at the position of the gate is cut after removing the optical element from the molds so as to be aligned with a predetermined position, the cutting position can be set at a position away from the transmission surface or the reflection surface. Therefore, the cutting operation can be performed without scratching the transmission surface or the reflection surface.
Furthermore, the optical element may be a lens, and the light-shielding zone may be used as a light-shielding mask for obtaining a super-resolution effect of the lens. For example, when the lens is applied to an optical pickup device, the diameter of the focussed spot can be reduced using a laser beam emitted from an inexpensive laser diode without shortening the wavelength of the laser beam, and the optical pickup device can be used to write or read of information on a high-density optical. disk.
According to another aspect of the present invention, there is provided a method for manufacturing an optical element having an optical axis by resin-molding using molds, wherein a gate is provided in the molds so as to correspond to the position of the optical axis in a transmission surface or a reflection surface of the optical element, resin is injected from the gate, and a light-shielding zone is provided at a predetermined portion of the optical element corresponding to the position of the gate.
Since the optical element is formed by the resin radially filled from the gate formed at the position of the optical axis, that is, filled with rotational symmetry with respect to the optical axis, an optical element which does not exhibit astigmatism and which does not have directional properties, unlike the conventional lens formed by filling resin in one direction, can be easily manufactured. When the optical element is a lens, for example, a lens capable of reducing the diameter of a focussed spot using the light-shielding zone without shortening the wavelength of light can be easily manufactured.
Furthermore, a roughened surface rougher than the transmission surface or the reflection surface may preferably be formed at the position of the gate in the molds, and the roughened surface may be transferred to the optical element at the position of the gate to form a light-shielding plane serving as the light-shielding zone.
With the above arrangement, only the roughened. surface, which is rougher than the transmission surface or the reflection surface, is formed in the mold at the position corresponding to the light-shielding zone of the optical element, so that the optical element can be easily manufactured at low cost.
Further objects, features, and advantages of the present invention will apparent from the following description of the preferred embodiments with reference to the attached drawings.